Scanning Electron Micrograph (SEM) showing detail of gene transcription from polytene chromosome in Chironomus sp. insect. Polytene chromosomes form when multiple rounds of replication produce many sister chromatids that remained synapsed together. In addition to increasing the volume of the cells' nuclei and causing cell expansion, polytene cells may also have a metabolic advantage as multiple copies of genes permits a high level of gene expression. Magnification: 50,000x when printed at 8x10".